Think of it like this: In FLCH, the system will ATTEMPT to make the level change within 125 secs, at the speed set in the MCP speed window. It will vary the thrust in an ATTEMPT to achieve that. Once the thrust reaches the limit (whether it be IDLE for a descent or CLB/CON for a climb) it can't do any better.
It's designed to reduce the thrust change required for small climbs or descents (ie a couple of thousand feet or so).